1. Field of the Invention
The field of the invention is that of signal processing for systems disposed in differential configuration. Differential configuration is understood to mean a configuration where two systems allow a variable representing a parameter and its opposite to be generated simultaneously.
This configuration allows certain errors to be avoided or to be significantly reduced. This method is particularly suited to the processing of signals coming from measurement sensors where the measurement may be both tainted by noise and be subjected to spurious effects like thermal drifts. The correction for thermal drifts is indeed a major problem in measurement systems. There exist numerous physical principles where a variable, an output signal and its opposite may be readily obtained. Resonant mechanical devices, certain optical devices, electrical or electronic devices will notably be mentioned. To give a simple example, if the movements of an object, which may be a plate, a beam or a membrane, are measured, a displacement of +d of a first side of this object and of −d of the other side is obtained.
2. Description of the Prior Art
The general case where a single system processes the information is presented in FIG. 1. The system S receives, as input, the signal y to be analysed. S returns the output signal u which is modulated by the input y. S also receives noise terms b which form spurious terms on the output u. A processing operation ST enables u to be demodulated in order to obtain a signal X varying according to a polynomial function with y, this variation law being obtained in an exact or approximate manner.
The following may thus be written:
X=X0ƒ(y) with ƒ(y) a polynomial equal to 1 for y=0, X0 forming the quiescent output of the processing operation, in other words for a zero input signal.
In order to determine y starting from X, X0 must be known together with the coefficients of f. These parameters are linked to the characteristic physical dimensions of S and may be determined by calibration, but they can vary as a function, for example, of temperature.
If, in order to determine y starting from X, the numerical values for X0 and the coefficients of f determined during the calibration, which was carried out at a temperature that may be different from the effective temperature of the system at the moment when the latter returns the information X, are simply used, an error is committed in the estimation of y which may be incompatible with the degree of precision required. Conventionally, this error is decomposed into two terms:                a zero-bias error corresponding to the error in the estimation of y for a zero input;        a scale factor error varying as a function of the value of the input.        
In addition, the noise present in the system S leads to noise in the estimation of y which may also be incompatible with the required precision.
Finally, in the case where the processing operation allowing X to be determined starting from u is a digital processing operation, the clock noise involved in the system sampling will cause additional noise in X.